The U.S. Congress has ordered that all cargo entering the country must be inspected for clandestine radiological and nuclear materials. Unfortunately, this is still not feasible. Although threat materials emit radiation (principally gamma rays and/or neutrons), sufficient shielding can greatly reduce the amount of escaping radiation. In addition, natural background radiation (cosmic rays and radioactive materials in the environment) further complicate detection. In the short time allocated for an entry scan at a shipping port (typically less than 2 minutes and often less than 1 minute), current detectors cannot detect a well-shielded nuclear weapon.
Advanced detectors with large solid angle acceptance and high detection efficiency are needed. In addition, detectors are needed that localize a source of radiation, preferably in three dimensions. Most backgrounds produce a broad uniform distribution of radiation. Therefore, a detector is needed that can distinguish a clandestine source from backgrounds according to particles coming from a single location.
What is needed is an advanced detector or detector array capable of efficiently detecting even a well-shielded source, separating neutrons and gamma rays of various energies, and determining the location of the source in three dimensions. Preferably the detection and localization may be completed rapidly and automatically, without using expensive or rare materials, and preferably at low cost.